Process control systems customarily comprise a multiplicity of individual control loops, each of which is an independent entity, which are interrelated to one another by means of a central processor. A typical control loop of the type suggested heretofore consists of a sensor, a signal transmitter, a set-point controller, a positioner, an actuator, and a valve.
The set-point controller is a comparatively expensive device the cost of which depends upon the number of modes of operation which are provided, the capability within each mode, and the overall capability and sensitivity of the controller. It functions to compare a signal which is derived from a loop sensor with another signal that represents a quantitative control setting that has been input either manually or from a central system processor. The relationship between the inputs and output of the set-point controller is ordinarily complex and variable, the complexity being a function of the number of modes which the controller is capable of bringing into computational capability, and the variability of the relationship being due to the rangeability, response, and ease of adjustment of each mode. The set-point controller responds to a discrepancy between the input signals thereto, to compute and transmit to the valve positioner a new valve setting which will reduce the discrepancy to zero. The output of the set-point controller is ordinarily a signal of fixed magnitude which represents a new valve position to which the valve should be set as quickly as possible to eliminate the discrepancy between the input signals to the set-point controller. The new valve position is normally transmitted to the positioner in the form of a standardly accepted voltage or current signal used in process control loops (typically a 4-20 ma dc signal or a 0 to 5 volts dc signal).
The valve positioner converts the output signal from the set-point controller, typically a 4-20 ma dc current signal, into a discrete valve position. To do this, the valve positioner converts the 4 to 20 ma current signal to a voltage signal, and then compares that voltage signal to a voltage which is derived from a potentiometer driven by the actuator output shaft. Any difference between these two voltage signals causes the actuator motor to be energized to rotate the shaft until the difference is reduced to zero, thereby changing the valve opening. With the new valve setting, the change in media flow causes the sensor output to change in a direction which matches the quantitative control setting at the set-point controller. As the sensor output varies with changing flow, the set-point controller constantly computes and transmits new valve positions to the positioner until a final position is reached when the output of the sensor matches the quantitative control input.
In general, the valve positioner is a linear device, i.e., equal increments of the input signal thereto cause the output shaft of the actuator to change in equal increments. A few options are customarily available in these known systems, e.g., electronic braking, and duty cycle and/or hunting control.
The lowest cost controllers presently available are usually single mode controllers with limited adjustment within that mode, while the highest cost controllers are usually three-mode controllers with each mode being highly variable and with little interaction between the adjustments. Regardless of the name given to each mode, it constitutes primarily a timing circuit and an associated analog decision circuit which cooperate to determine when and for how long a particular mode function will be activated each time a control action is required of the set-point controller. The controller normally operates in accordance with a plurality of algorithms which determine when, how long, and how fast various parameters should be combined in a mode function, and the controller is tuned by adjustment of these parameters to achieve a desired response in a control loop and to minimize loop upsets by matching the controller's response to the inherent dynamics of a particular process loop. In any event, known set-point controllers are normally extremely expensive pieces of equipment, with a large portion of the costs of the controller being attributable to the type of circuitry employed therein, i.e., analog circuits which operate on infinitely small input voltage variations and which tend to be very critical of component selection and circuit gain and balance adjustments and which, over the normal life of the controller, are subject to drift due to temperature, voltage and humidity variations and component deterioration whereby the finely tuned set-point controllers typically employed heretofore required constant observation and maintenance to maintain high loop control efficiency.
The present invention is directed to an improved controller which is far simpler and less costly than set-point controller/positioner combinations suggested heretofore, but which nevertheless can provide a desired process control function reliably and accurately. In contrast to set-point controller/positioner combinations suggested heretofore, the system of the present invention eliminates the need for a valve positioner and, instead, provides an output signal which is used directly to control the valve actuator. In addition, in contrast to the prior art systems described above, which operate to produce a signal that designates a specific position to which the valve used in a process control loop should be set, the system of the present invention operates in accordance with an entirely different and unique philosophy, i.e., the system simply monitors the magnitude of a particular parameter in the process control loop, compares it to a specified value which that parameter should have, and actuates the valve to cause a change in a flowing fluid medium while the parameter continues to be monitored until such time as the parameter being monitored reaches the specified or desired value. In short, the system of the present invention does not concern itself with valve position. Instead, the system of the present invention operates, without regard to the existing valve position, to cause the valve to move in the proper direction and at a proper rate needed to provide corrective flow action, with said corrective flow action being accomplished slowly enough so as not to inject process disturbances, but fast enough so that the corrective action does not always lag behind a required flow change.
Since the system of the present invention does not concern itself with valve position per se, there is no need for a position feedback in the actuator, or with a positioner which provides a position comparison function. Moreover, since valve position is disregarded in the present invention, the complex mode functions which have typically been employed in controllers heretofore, and which function simply to modify an originally designated position when the controller compares the sensor input to the set-point input, can be eliminated. The resultant circuit which characterizes the present invention is accordingly far less costly than controllers which have been suggested heretofore, and can be made in such small sizes that, although the controller can be located at a position remote from the actuator as has been the case heretofore, it can also, if desired, be mounted directly adjacent the actuator or incorporated into the actuator housing.